The Habit
by writeforlove
Summary: The first few times it happened, Derek managed to shrug it off. But the night after he's left at the alter, Derek finally had to ask. "So, stealing my girlfriends is habit forming?"


Derek was rendered speechless by the words flowing from his girlfriend's mouth. Well, ex-girlfriend, as of this conversation, apparently.

Derek struggled to find something to say, but he came up empty again, and could only reflect on how unexpected this all was. He really hadn't seen this one coming at all, and he supposed he really should have.

After all, Casey had told him it was coming only two days previously, only he hadn't taken her seriously as she was drenched in green slime (and, yeah, maybe that was his doing, but could you blame him?) and screeching at the top of her lungs that he had just ruined her date and she was going to get even be taking his from him (apparently Casey _could_ blame him. Shocking, huh?).

And yet, somehow she had accomplished it, and so here he was, sitting on the edge of his bed while Emily Davis broke up with him. And if that wasn't enough of what the _heck_ moment, Derek was still puzzling over how exactly Casey had achieved it. He knew that Emily was immune to Casey's whining about his failings after so many years, so that couldn't have been it.

Maybe she had been really underhanded. Could she have convinced Emily that Derek was cheating on her somehow? Derek wasn't sure Casey had the balls to do that, and he couldn't think what other method she might have used. So, seeing no alternative, Derek looked up into Emily's tear streaked face and asked the two million dollar question. "Why?"

"Oh, well, Derek there's someone…else," she said, watery brown eyes shifting to the floor. "I've met- well not really _met_, but more got together with- someone else."

Derek's mind was reeling. How exactly had Casey managed to make a cheater of her best friend? And how the hell had she managed to silence her conscience enough to let herself pull it off?

But even as his mind focused on Casey, his mouth was on auto-pilot. "Who?"

Now Emily looked really panicked. "I can't tell you! She-they'd be furious with me!"

_She_? Derek thought, aghast. And then, suddenly it clicked.

.LWD.

Leaning in Casey's doorway a little while later, Derek clapped slowly. She looked up, shooting him the same innocent look as always, only now, it seemed a bit false to him.

"Bravo," Derek said, "You did it. You seduced my girl friend away from me. I might be angry, but, well, the idea is way too hot, I can't even bring myself to care that Emily just broke up with me."

Casey tried for about a second to look horrified, succeeding far better than Derek was really comfortable with. Soon enough, though, she let out an evil little giggle. "I know, genius, right? And it was a lot easier than I thought it would be, if she's already broken up with you."

Derek said nothing, and focused on keeping a smirk on his face, even though he felt like he was sort of in shock.

"I mean, it only happened the one time, I thought it would take at least a second _kiss_ to convince her to dump you. She was crushing on you for years, and yet…" Casey grinned evilly, and when Derek still said nothing, she shrugged, "Maybe I'm the better kisser, is all?"

Derek cocked his head to the side. "Is that what you think that was about?"

"What?" asked Casey sharply, annoyed that her gloating was being interrupted.

"You think this is about proving who the better kisser is?"

"No," Casey said, "This was about taking something away from you because you took something from me."

"Oh, I don't know about that," Derek said, "If that's what this was, then you would have taken something that matters more to me than Emily ever did."

"Oh, I don't know," Casey countered, "I think Emily probably mattered to you about as much as one date with a random guy mattered to me. Plus, I think that the fact that your girlfriend ditched you for a _girl_ will catch up with you, eventually."

"Maybe," Derek said, grinning again, "But right now, I just can't help picturing the two of you going at it in high heels and lacy underwear."

.LWD.

So when it happened, that first time, Derek let it roll off his back pretty easily.

What he had told Casey, had been true. The mental image of Emily and her best friend had been more than enough to make up for the loss of a relationship that had been beginning to bore him anyway. And, he'd figure, initially, that what Casey had done would probably screw up her friendship with Emily, which meant a lot more to her than Derek's relationship with the same girl had ever meant to him. And even when, a couple of weeks later, it seemed like Casey had somehow managed to keep things basically the same with her best friend, Derek wasn't all that bothered.

One month into the last summer before university, however, he was starting to get annoyed.

As usual, he had no compunction with sharing that annoyance with his dear step-sis. "Anna just broke up with me," he said accusingly, leaning in Casey's bedroom doorway.

"Really? That's too bad," Casey replied innocently. This time, though, Derek knew right away that she was faking it.

"I thought you said the Emily thing made us even?" Derek questioned, a bit bitterly. He'd been dating Anna since just before school let out, and he had been fairly certain she'd been about ready to sleep with him. The waste of effort Casey had caused irked him.

"Well, it did," Casey acknowledged, "But you remember Joel, right? That guy I was with when you dumped the vat of slime on me? Well, I found out that after I went out with him, he hooked up with Natalie Greenberg, and now she's pregnant and they're getting married. And so, I figured I owed you one for getting me clear of that whole mess."

Derek was silent for a moment, wondering if it was just him, or if there was a huge flaw in her reasoning. "You owed me one, so you seduced my girlfriend in to breaking up with me? _For the second time?_"

"I was doing you a favour," Casey insisted. "I only knew about Natalie because Anna told me about running in to her at the free clinic downtown. And when I asked Anna what she was doing at the clinic…well, as a second favour, because I'm feeling so generous, I'm going to spare you her answer."

Derek thought about it a moment. He nodded once and left, knowing that Casey was smiling…no, _smirking_ at him behind his back.

..LWD.

So, the second time, Derek let it go again. He didn't even question the idea that Casey had done it as a favour to him.

After all, it was just the sort of favour that typified the way he and Casey related to one and other. She acknowledged that he had (inadvertently) helped her out, and she had (indirectly) paid him back in the most annoying way she possibly could have, while still doing him a legitimate favour. It was the sort of situation where it was impossible to declare a clear winner, though Derek suspected that maybe she had scored a few more points this go around.

But, the third time it happened…well, even the mental picture of it wasn't enough to soften Derek's anger.

"Okay, Case, you need help," he started in, once again leaning on the door frame of her bedroom, only this time the bedroom was located in Kingston rather than London.

"You're right," she said, glancing up. "Pass me those scissors, would you? And maybe you could start wrapping that stack over there."

Derek looked around in confusion. "I didn't mean you needed help _wrapping Christmas presents_," he said, supremely frustrated, "I mean, you need help for the extreme mental disorder you clearly possess."

"Derek, half these presents are from you, you know. Look, I picked up a book of old jokes for Edwin from the both of us," Casey said sternly, retrieving the scissors herself.

"Do you not hear me, Crazy?" he replied insistently, "You're clearly unbalanced. Do you realise, Shelia is the _third_ girlfriend of mine that you've seduced into leaving me?"

Casey finally stopped wrapping the large blue fluffy thing that Derek presumed was destined one of the youngest Venturis, either Marti or Simon. She looked up at him and sighed. "Okay, I'll admit it, this one was on me."

"Huh?" said Derek faintly.

"Look, Derek, she came here last week, chattering on and on about how great you are and how well your relationship was going and how serious the two of you were getting," Casey explained plaintively, "And I tried to play nicely, I swear I did. I sat here, working on my essay while she prattled on. I fed her cookies and hot chocolate and let her think that you and I were the kind of siblings who are friends with each other's girlfriends and boyfriends. I just gritted my teeth and got through the day, which I think I did splendidly, if I do say so myself. Shelia went away smiling, at any rate."

"I'm not seeing any explanation for your craziness," Derek said impatiently, because he was pretty sure that he had never let Casey get that many words out in a row before, without interrupting.

"I'm getting to that. Anyway, I graciously dealt with your girlfriend for three and a half hours, and then I closed the door behind her, hoping never to see her again by myself. But you know what, Derek? She came back the very next day. And the next day, and the day after that…Derek, I couldn't shake her! She was everywhere, wanting to be my best friend and my shopping partner and my study buddy! There was no escaping from her!"

"Huh," said Derek, "I did start to wonder why she was hanging around so much less this last little while."

"Well, now you know," Casey said testily, "And I'm sure you see why I had to resort to this sort of tactic (again). I had to be sure that she'd leave the both of us alone, for good. I was really beginning to worry that you'd bring her home for Christmas and I'd never get any peace!"

..LWD.

So, when it happened the third time, somehow, Derek bought Casey's excuse and, with a bit of concentration, he managed not to hold the experience against her.

They went home for Christmas with the pile of presents wrapped by Casey, and Derek pretty much forgot about Shelia. He saw her once, as he and Casey were walking towards his car, headed out to find a birthday present to mail to Marti. Shelia had stopped dead in her tracks for a moment, staring at Casey for a moment before darting a guilty look at Derek and hurrying away.

Despite the lack of hard feelings, though, Derek was careful to keep any girl he was interested in well clear of Casey from them on. In fact, he had been dating Katie for nearly two months before the blonde girl even found out he had a stepsister living on the same campus. He might have succeeded in keeping the two girls apart even longer, if Casey weren't so damn organised.

As it was, Derek's dear stepsister broke into his dorm room the week before they were to leave Kingston for the summer and started packing his things. Derek probably would have been more or less okay with that, since it meant he didn't have to pack up himself, but Casey was in the middle of emptying out his closest when he and Katie arrived back at the dorm after dinner.

"Derek, there's a girl in your room," Katie complained when she spotted Casey.

Before Derek got the chance to diffuse the anger that was clear in his girlfriend's voice but pointing out that it wasn't a girl in his room, it was _Casey_, his stepsister was stepping forward. "Oh, hi! I'm Casey, Derek's stepsister. Don't mind me, I'm just packing his things so that we can actually leave for home promptly this time."

"You have more than one stepsister?" Katie asked, clearly unimpressed.

"Oh, so Derek's told you a bit about the family?" Casey said enthusiastically.

"Well, yeah, but he never mentioned you," the blonde girl replied, "Not once in two months did Derek mention he had any family around here."

"Oh, don't mind that so much," Casey said blithely as Derek sank down onto a chair, since he clearly wasn't required for this conversation. "Derek and I usually try to pretend we don't know each other. Sorry, but what was your name?"

"Katie," the girl answered, frowning a bit, "Derek didn't mention me to you?"

"No, he didn't," Casey said, and suddenly both girls were looking at Derek. When he failed to come up with any sort of response to their glares, Casey huffed.

"Typical Derek," she said, "Well, I suppose I'll leave you two alone now, and finish packing another time."

"No, no," Katie said quickly, "I'm just going to leave as well."

"I'll take you home," Derek said quickly, knowing at least better than to argue that she shouldn't go at all.

"I suppose-"

"Don't be silly," Casey said, "I can give you a lift, Katie. I'm on my way out, anyway."

And somehow, when the two girl left a moment later, Derek got the feeling that Katie would be breaking up with him in the very near future.

Still, he didn't anticipate that he would be girlfriend-less by noon the next day. None-the-less, when he found himself in just that state, Derek had headed straight over to Casey's dorm.

"So, I haven't pulled a prank on you in weeks," Derek said the moment he had her door open. "Which I guess means you did it this time as some sort of favour to me, right?"

"No," Casey said as she neatly labelled one of the many boxes in her dorm room, "I really like Katie. She's much too good a person to be with someone like you. This time, it was a favour to her."

"You seduced Katie into breaking up with her boyfriend, for her own good?" Derek asked sceptically.

"Uh-huh," Casey said as she began sorting her textbooks into piles.

"I think," said Derek, "That I am going to require some details on this one. I'm beginning to question how you keep stealing my heterosexual girlfriends so easily."

"Well, stealing them isn't really the objective, now is it?" asked Casey, "All I'm trying to do is get them to leave you. Shelia was easy, one kiss and she was so freaked out she still runs away whenever I see her on campus. Katie… well, she's a different story, and not one that I'm prepared to tell you. Let's just say, Katie took a good deal more persuading. So much so, I probably wouldn't have bothered if it weren't in her best interest."

That, apparently, was enough for Derek. As much as he wanted to, he really had trouble staying angry with Casey when he kept getting distracted by what 'a good deal more persuading' could possibly mean.

Even so, when Katie came to him at the start of next semester, saying that she wanted to get back together with him after a summer spent doing a lot of soul-searching, Derek turned her down. He didn't know if he believed that not being with him was better for Katie, but certainly it would be better for the blonde not to be exposed to Casey again.

..LWD.

Derek refused to keep count of the number of girlfriends Casey had lost him after the whole Katie incident. He quickly figured out the warning signs to look for whenever one of his dates first met his stepsister, and was sometimes able to circumvent an embarrassing breakup by ditching the girl shortly after introducing her to Casey, but that was as good as it got, until one day he realised that six weeks had gone by since Casey met Melinda, and by some miracle, the girl was still taking his calls.

He didn't know what to make of it, until Casey sought him out one night in his apartment. As it happened, Melinda had just left to go catch some sleep in her own apartment before her midterm the next day. Derek had not planned on getting up from his bed until a knock came at the door. Soon enough, he found himself sat down in the living room by his stepsister.

"You need to break up with Mel," she said, standing between Derek on the couch and the TV.

"Oh, no you don't," Derek said, "If I've finally found a girl that refuses to succumb to your weird little game, then I am most certainly not going to dump her."

"Here," Casey said, handing Derek a piece of cloth.

He took it, and groaned when he saw what it was. "Oh, come on," he groaned, flinging a pair of familiar panties back at the girl who had handed them to him. "Mel just left! How did you possibly have time to get these?"

"I've had those since this afternoon," Casey said.

"You seduced her this afternoon, and then she came over here to…"

"No," said Casey, "I seduced her four and a half weeks ago. That's why you have to break up with her."

"Oh?" he said weakly.

"Yeah, what kind of person cheats on their boyfriend with his stepsister and then keeps dating him for more than a month without telling him? I knew there was something off about Mel the moment I met her, and I was right. The girl has no morals!"

Derek thought it might be just a bit much for Casey to be accusing anyone else of loose morals right then, when she still held the underwear that she had pulled from the body of her stepbrother's girlfriend. He thought about saying as much, but in the end, he sighed and settled with breaking up with Melinda.

..LWD.

Derek could admit that had gotten a bit paranoid after that. He started to try to pick girlfriends based on a convoluted set of criteria based on Casey's opinions, when he picked them at all. Mostly, he just didn't date.

That changed, however, when he met Staci. He recognised something in her that was unlike any of his previous girlfriends, and he seized upon the relationship because of the fragile hope that Casey wouldn't want to get rid of someone who was so much like her. None-the-less, Derek went to great pains to ensure that Casey and Staci didn't spend any time together alone. This goal was helped along greatly when Casey took the opportunity to spend two semesters studying in Europe.

And, somehow, Derek managed to date Staci for a year. In that time, she'd met Casey twice, briefly and yet somehow remained his girl friend. Taking that as a sign, Derek promptly asked her to marry him.

She said yes, which actually increased Derek's fears about the relationship to astonishing levels. Thus motivated, he managed to keep Casey and Staci apart right up until the day before the wedding. That was when Casey arrived home, master's degree in hand, and primed to ruin Derek's life once again. Even so, Derek might have stood a chance, if he'd only been able to convince Staci to remain at his side constantly until the actual vows were exchanged.

Somehow, though, his bride-to-be was whisked away from him by a swarm of MacDonald-Venturi girls who were looking to spend time with their prospective step-daughter/sister-in-law.

And after that, Derek just knew he would find himself facing off against Casey once again. He supposed he should be angry, leaning in the doorway and looking at the girl that had caused him to be jilted at the alter, but he wasn't. Derek felt resigned and maybe a bit morbidly curious, but he had trouble mustering any real anger.

"So, what was it, this time?" Derek asked. "Staci's own good or mine?"

"Um," Casey began, turning to face her step-brother but refusing to meet his eyes.

"Or are you just gay or what?"

"Uh, no," Casey said quietly, "I am not a lesbian."

"Alright, then, let's have your grand excuse for destroying yet another relationship of mine by sleeping with my fiancé," Derek said.

"Actually, I don't have an excuse this time," Casey said weakly, staring at the carpet of her old bedroom in the Venturi-MacDonald home. "This time was just…habit, I guess."

"Stealing my girlfriends is habit forming?" parroted Derek in slight disbelief. "You have formed a habit of stealing my girlfriends."

"Stop repeating me," Casey snapped suddenly, now able to meet Derek's eyes.

"Well… I guess there's only one thing to do for it," Derek said finally, taking the necessary steps to cross the room and pull Casey to him. She flinched as if she expected some sort of attack, and then wasn't prepared when the assault came in the form of his lips on hers.

A moment later she broke away enough to gasp, "Derek, what are you doing?"

"Well, I figure you're the only girl who isn't going to break up with me when you inevitably have sex with you," Derek explained, not easing his grasp on Casey's body in the least.

"That's…logical, I suppose," Casey replied after a moment, before looking sternly at Derek, "So long as I'm not reduced to having sex with myself too often."

"It's a deal."


End file.
